1. Field of the Invention
The present invention relates generally to the field of computer systems and, more specifically to computer systems including a method, system, and apparatus for alleviating router congestion when the router is processing packets transmitted by computer systems having a congestion notification capability.
2. Description of the Related Art
When TCP/IP is used for data transmission between computer systems, the transmitted data is routed through intermediate routers. These routers may experience congestion. When the router is severely congested, the router will drop packets indiscriminately in order to reduce the congestion and to indicate to the sending computer systems that the router is congested.
When the router is only moderately congested, a method of congestion notification has been proposed which requires the marking of packets to indicate congestion to the sending computer systems. An addition to the TCP/IP protocol, called Explicit Congestion Notification (ECN), has been proposed as a method of indicating congestion to the sending computer systems. The ECN describes a method of marking packets in order to provide an indication of moderate congestion prior to the router actually dropping packets. Each packet includes an ECN bit and a Congestion Experienced (CE) bit in the IP header of the packet. The ECN bit may be set by the sender to indicate that the sender and receiver of this packet have the ECN capabilities. The CE bit is set by the router through which a packet passed when the router is experiencing moderate congestion.
When a router which utilizes the ECN protocol is moderately congested, the router will mark and pass packets having the ECN bit set, and will drop packets which do not have the ECN bit set. Therefore, packets transmitted by senders which have the ECN capability receive preference over packets transmitted by senders which do not have the ECN capability.
When senders are notified that a router has become moderately congested, the senders should either cease or slow transmissions in order to relieve the congestion. If a sender without the ECN capability continues to transmit packets, these packets will be dropped. If a sender having the ECN capability continues to transmit packets, its packets will not be dropped until the router becomes severely congested. Therefore, it is possible for senders having the ECN capability to abuse the ECN protocol by continuing to transmit packets even after being notified that the router is moderately congested.
Therefore, a need exists for a system, method, and product for reducing preferential treatment given to packets transmitted by computer systems having a congestion notification capability.